Deep bed media filtration is used to remove fine oil droplets and solids from oil-contaminated water before disposal, or to remove particulate material from water prior to use in, for example, waterflood injection. After a filtration cycle, the bed is loaded with contaminants (e.g., oil and solids) and loses its effectiveness. The bed can further clog thereby preventing fluid from passing through the filter system. Notably, contaminant-loading causes pressure drops across the bed and may lead to a fracture in the filter media. Fractures allow oil and solids to bypass the filter bed matrix and contaminate the outflow. Depending on the filter media, the media can be replaced (i.e. when the media is activated carbon or modified clay) or backwashed to remove contamination (i.e. when the media is sand or ground nutshells).
Recyclable or regenerable filter media (commonly ground nutshells) is preferable in view of cost and waste generation. These media beds are periodically backwashed to remove the dirt and contamination from the filter media; the dirt and contamination is then disposed or reclaimed.
The backwash process is, as the name suggests, a reversal of the flow accompanied by an agitation of the filter media to “shake loose” the adhered contaminates. In the case of oil-contaminated water, the process is understood to be facilitated by the de-agglomeration of the filter media from oil droplets. The oil, loosely bound to the surface of the filter media and surrounded by multiple particulates of the filter media, is then carried out of the filter system for recycling or disposal.
The oil removal efficiency of regenerable filter media is dependent on the flux of the contaminated water through the filter bed (generally reported as gpm/ft2) and the oil-droplet size (generally reported as turbidity). Due to the mechanism of oil adhesion, ground nutshell media is limited to low flux rates and large oil-droplet sizes (higher turbidity). Despite over fifty years of industrial use, improvements to regenerable filter media have been elusive.